Naval, scientific and commercial marine operations often involve deployment of a submersible payload suspended by a suitable cable or rope extending from an overboarding system on a ship or other surface vessel. Various types of suspension elements, depending upon the mission, include synthetic and wire rope, electromechanical and optomechanical cable, and the like, that may be attached to a suitable ship or surface vessel overboarding system for deployment of a submersible payload. Ship overboarding systems useful in practice of the present invention include cranes, winch and overboarding sheave combinations, as well as A-frame and U-frame overboarding systems. Numerous types of submersible payloads are routinely employed, with the specific payload again depending upon the mission involved. These submersibles include remotely operated vehicles (ROV), with and without cages, conductivity, temperature, density (CTD) scientific instrument assemblages, camera sleds, and the like.
The method of operation for various submersible payloads may involve towing, casting, holding a fixed elevation in the water column or deploying the payload on the ocean floor at a fixed site. In each situation, it is generally desirable to remove, neutralize or, at least, minimize the wave induced motion of the surface vessel from the motions of the submerged payload. Otherwise, induced motions in the submerged payload create difficulties in measuring, viewing, photographing, recording or collecting useful data by the submersible. Also, it is well recognized that ship motion can lead to sudden jerks that result in broken or snapped suspension cables caused by momentary slack development in the cable, followed by sudden removal of the slack, to produce instantaneous stretching and loading of the cable.
Various attempts to isolate submersible payloads from the motion of the surface vessel have included the use of both active and passive motion compensation systems.
Active compensation systems involve paying out, and hauling in, cable in response to motion sensor devices, such as one or more accelerometers attached to the ship or to the overboarding sheave block. This approach requires continuous acceleration and deceleration of the cable winch over time periods of only several seconds duration. Such rapid and reversing actions require drivers or prime movers of large horsepower due to the large inertia of the winching system.
Also, for efficient operations, active systems must be provided with accurate, instantaneous data on either the vertical position or velocity of the ship. This data must be available essentially on a continuous basis and serves as the intelligence for commanded winch rotations. Unfortunately, the accelerometers provide acceleration data only and, to obtain either position or velocity, mathematical integration of the accelerometer signal is required. The mathematical integration leads to difficulties due to lack of knowledge of the initial conditions relative to velocity and/or position, signal error, and effects of outside influences, such as extraneous vibrations of the accelerometer(s) mounting structure and the roll and pitch of the ship.
Passive compensation systems are usually based on the use of air or gas springs to provide a compliant member in the overboarding system. This member may be a bobbing crane boom that holds an overboarding sheave or it may be a ram tensioning type device designed to maintain nearly constant tension on the suspension cable.
The present invention differs from the active and passive motion compensation devices described hereinabove and from any other known motion compensation system.
Accordingly, it is an object of the present invention to provide a new and novel motion compensation and tension control system for a surface vessel supporting a submersible payload.
Another object of the present invention is to provide a motion compensation and tension control system that is positioned between the submerged end of a support cable and a submersible payload.
An additional object of the present invention is a motion compensation and tension control system that essentially isolates a submersible payload from the wave induced motion of the surface ship.
A further object of the present invention is a motion compensation and tension control system that prevents or minimizes wave induced ship motion from moving or disturbing a submersible payload.